1. Field of the Invention
The present invention relates to a thread groove pump mechanism of a vacuum pump, a vacuum pump including the thread groove pump mechanism, and a rotor, an outer circumference side stator, and an inner circumference side stator used in the thread groove pump mechanism and, more particularly, to a thread groove pump mechanism of a vacuum pump usable in a pressure range of medium vacuum to ultra-high vacuum, a vacuum pump including the thread groove pump mechanism, and a rotor, an outer circumference side stator, and an inner circumference side stator used in the thread groove pump mechanism.
2. Description of the Related Art
When a semiconductor device such as a memory or an integrated circuit is manufactured, it is necessary to perform doping and etching on a high-purity semiconductor substrate (wafer) in a chamber in a high vacuum state in order to avoid influence due to dust and the like in the air. A vacuum pump such as a turbo molecular pump is used for exhaust in the chamber.
As a vacuum pump used conventionally, there is known a combination pump including a turbo molecular pump mechanism A and a thread groove pump mechanism 200 provided below the turbo molecular pump A as shown in FIG. 10. The thread groove pump mechanism 200 includes a substantially cylindrical stator 202 disposed coaxially with a substantially cylindrical casing 201, a rotor 203 including a rotor shaft 203a turnably supported coaxially with the stator 202 and a substantially cylindrical cylinder portion 203b disposed between a casing 201 and a stator 202, and a plurality of thread groove portions 204 engraved on the inner circumferential surface of the stator 202 opposed to the cylinder portion 203b. 
In such a vacuum pump, when a gas exhaust flow rate of gas compressed by the thread groove pump mechanism 200 increases, the exhaust performance of the thread groove pump mechanism 200 tends to be deteriorated. Therefore, as a vacuum pump for improving the exhaust performance of the thread groove pump mechanism, there is known a vacuum pump including a co-flow thread groove pump mechanism including an outer-circumference-side thread groove portion provided between a stator and a cylinder portion and an inner-circumference-side thread groove portion provided between the stator and the cylinder portion (see, for example, Japanese Utility Model Application Publication No. H5-38389).
In the vacuum pump including such a co-flow thread groove pump mechanism, gas transferred into the thread groove pump mechanism is distributed to an outer-circumference-side thread groove portion 90 and an inner-circumference-side thread groove portion 91 provided in parallel in a pump radial direction R as shown in FIG. 11. According to a drag effect due to high-speed rotation of a cylinder portion 92 relative to a casing 93 and a stator 94, the gas in the outer-circumference-side thread groove portion 90 and the gas in the inner-circumference-side thread groove portion 91 are transferred from an intake side to an exhaust side in an up-down direction H while being respectively compressed.
If the exhaust performance and the compression performance of the outer-circumference-side thread groove portion 90 and the exhaust performance and the compression performance of the inner-circumference-side thread groove portion 91 are equivalent, the gas exhaust amount of the outer-circumference-side thread groove portion 90 and the gas exhaust amount of the inner-circumference-side thread groove portion 91 are equal and the outlet pressure of the outer-circumference-side thread groove portion 90 and the outlet pressure of the inner-circumference-side thread groove portion 91 are equal. Therefore, the co-flow thread groove pump mechanism in which the outer-circumference-side thread groove portion 90 and the inner-circumference-side thread groove portion 91 are provided in parallel can exhibit double compression performance compared with a thread groove pump mechanism in which only one row of a thread groove portion is provided.
However, in the vacuum pump, a rotation radius of the gas in the inner-circumference-side thread groove portion 91 is smaller than a rotation radius of the gas in the outer-circumference-side thread groove portion 90. A centrifugal force acting on the gas in the inner-circumference-side thread groove portion 91 according to high-speed rotation of the cylinder portion 92 is smaller than a centrifugal force acting on the gas in the outer-circumference-side thread groove portion 90. Therefore, as indicated by arrows I in FIG. 11, a part of the gas in the inner-circumference-side thread groove portion 91 easily flows back from the exhaust side toward the intake side in a gap between the cylinder portion 92 and the stator 94. Therefore, a gas exhaust amount Q2 of the inner-circumference-side thread groove portion 91 markedly decreases compared with a gas exhaust amount Q1 of the outer-circumference-side thread groove portion 90. The gas exhaust amount Q1 of the outer-circumference-side thread groove portion 90 increases by an amount of the decrease in the gas exhaust amount Q2. It is likely that the exhaust performance and the compression performance of the co-flow thread groove pump mechanism are deteriorated.
The circumferential speed of the gas in the inner-circumference-side thread groove portion 91 is lower than the circumferential speed of the gas in the outer-circumference-side thread groove portion 90. A channel of the inner-circumference-side thread groove portion 91 is shorter than a channel of the outer-circumference-side thread groove portion 90. Therefore, the outlet pressure of the inner-circumference-side thread groove portion 91 is sometimes smaller than the outlet pressure of the outer-circumference-side thread groove portion 90. Consequently, a pressure difference occurs between the outer circumference side and the inner circumference side of the pump radial direction R near an outlet of the thread groove pump mechanism. It is difficult for the inner-circumference-side thread groove portion 91 to compress and exhaust the gas. It is likely that the exhaust performance and the compression performance of the co-flow thread groove pump mechanism are further deteriorated.
Specifically, in the intake side of the thread groove pump mechanism, as indicated by a back pressure characteristic shown in FIG. 12, since fluctuation occurs in exhaust side pressure (back pressure) between the outer circumference side and the inner circumference side, the gas divided to the outer-circumference-side thread groove portion 90 and the inner-circumference-side thread groove portion 91 and respectively compressed at the same intake side pressure flows back from the outer-circumference-side thread groove portion 90 or the inner-circumference-side thread groove portion 91 having smaller exhaust side pressure to the outer-circumference-side thread groove portion 90 or the inner-circumference-side thread groove portion 91 having larger exhaust side pressure. It is likely that the exhaust performance and the compression performance of the thread groove pump mechanism are further deteriorated.
Further, when the co-flow thread groove pump mechanism and a thread groove pump mechanism including one row of a thread groove portion capable of exhausting gas at a flow rate same as the flow rate of the co-flow thread groove pump mechanism are compared, the exhaust side pressure of the former tends to markedly rise. In a higher back pressure region than pressure P in FIG. 13, the exhaust performance of the former is lower than the exhaust performance of the latter. The inner-circumference-side thread groove portion does not function.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background.